CentOS 8: add docs and release note

Adds a support matrix page to documentation.

Change-Id: Ie42900d89afff40b8a12364b38de9854ed817275
Story: 2006574
Task: 39552

doc/source/index.rst

@@ -32,6 +32,7 @@ Contents
    :maxdepth: 2
 
    architecture
+   support-matrix
    installation
    usage
    configuration/index

doc/source/installation.rst

@@ -18,6 +18,9 @@ control host:
 - CentOS 8
 - Ubuntu 16.04
 
+See the :doc:`support matrix <support-matrix>` for details of supported
+Operating Systems for other hosts.
+
 To avoid conflicts with python packages installed by the system package manager
 it is recommended to install Kayobe in a virtualenv. Ensure that the
 ``virtualenv`` python module is available on the Ansible control host. It is

doc/source/support-matrix.rst

@@ -0,0 +1,24 @@
+==============
+Support Matrix
+==============
+
+Supported Operating Systems
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Kayobe supports the following host Operating Systems (OS):
+
+.. note::
+
+   CentOS 7 is no longer supported as a host OS. The Train release supports
+   both CentOS 7 and 8, and provides a route for migration.
+
+* CentOS 8
+
+Supported container images
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+For details of container image distributions supported by Kolla Ansible, see
+the :kolla-ansible-doc:`support matrix <user/support-matrix.html>`.
+
+For details of which images are supported on which distributions, see the
+:kolla-doc:`Kolla support matrix <support_matrix>`.

releasenotes/notes/centos-8-12073e91a157d0a2.yaml

@@ -0,0 +1,16 @@
+---
+features:
+  - |
+    Adds support for CentOS 8 as a host Operating System and base container
+    image. This is the only major version of CentOS supported from the Ussuri
+    release. The Train release supports both CentOS 7 and 8 hosts, and provides
+    a route for migration.
+upgrade:
+  - |
+    CentOS 7 is no longer supported as a host Operating System or base
+    container image. CentOS users should migrate to CentOS 8. The Train release
+    supports both CentOS 7 and 8 images, and provides a route for migration.
+  - |
+    Some images were supported by CentOS 7 but lack suitable packages in CentOS
+    8, and are no longer supported for CentOS. See `Kolla release notes
+    <https://docs.openstack.org/releasenotes/kolla/ussuri.html>`__ for details.